How to be two places at once: map datums

So you have a fancy new GPS unit, satellites fixes streaming in, and not a tree or building to get in the way. Your GPS does it’s fancy black-box magic and *bang* you’ve got a lat/lon for the point on the earth right under your feet. Pretty cool.

Now you look up the coordinate on your map. The point turns out not to be under your feet, but 200 yards away in the neighbor’s swimming pool. Huh? How are your feet in one place and the same map location completely different? Most likely your GPS and map are using different map datums.

The Earth is round. Learned that one in kindergarten, right? Well, it’s not. Not precisely round, that is. The Earth is actually a somewhat elongated lumpy ball of rock. In order to translate a geographic coordinate (or a whole map) onto the surface of the lumpy ball, you need a map datum.

A datum basically defines the position of the Earth’s surface relative to the center of the Earth. Initially these datums and their underlying ellipsoid models (fancy mathematical models describing the shape of the earth) were established by series of stations surveyed out from a central control point. A common datum used in the United States is the North American Datum of 1927, or NAD27. Not so coincidentally most USGS topographic maps made before 1985 used the NAD27 for a base datum.

As satellite technology and observations became available in the 1960s and 1970s, new “global” mathematical models (and datums) were developed that began to supercede the earlier models. The World Geodetic System of 1984 (WGS84) is now the standard for GPS technology worldwide, and in North America the North American Datum of 1983 (NAD83) was created to replace the aging, and somewhat innacurate NAD27.

Any good map should tell you what datum it’s based upon. Since your GPS is actually a fancy little computer, you can set it to the same datum as your map. Most GPS units are pre-set to WGS84, which is for most practical purposes identical to NAD83.

When you order a map from MyTopo.com, you have the flexibility to choose traditional NAD27 or the newer NAD83, and the datum of your map is printed right in the legend.

Voila, you’re back where you started.